1. Field of the Invention
The present invention relates to a high-strength steel plate which is used as a structural member of a construction machine or an industrial machine, has excellent delayed fracture resistance and weldability, has high strength of a yield strength equal to or greater than 1300 MPa and a tensile strength equal to or greater than 1400 MPa, and has a plate thickness equal to or greater than 4.5 mm and equal to or smaller than 25 mm; and a producing method therefor.
2. Description of Related Art
In recent years, with the worldwide construction demand, the production of construction machines such as cranes and concrete pumping vehicles has increased, and simultaneously, the size of these construction machines has continued to increase. In order to suppress an increase in weight due to the increase in size of the construction machine, demand for a lightweight structural member has increased, so that a change to high-strength steel having a yield strength of 900 to 1100 MPa-class is taking place. Recently, demand for a steel plate for a structural member having a yield strength of 1300 MPa or greater (and a tensile strength of 1400 MPa or greater) has increased.
In general, when the tensile strength increases over 1200 MPa, there is a possibility that delayed fracture due to hydrogen may occur. Accordingly, in particular, a steel plate having a yield strength of 1300 MPa-class (and a tensile strength of 1400 MPa-class) requires a high delayed fracture resistance. In addition, the steel plate that has a high strength is disadvantageous in terms of usability such as bending workability and weldability. Therefore, the steel plate requires usability that is not lower than an existing high-strength steel of 1100 MPa-class.
As a technique related to a steel plate for a structural member having a yield strength of 1300 MPa-class, a producing method for a steel plate which has a tensile strength of 1370 to 1960 N/mm2-class and has excellent hydrogen embrittlement resistance is disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. H7-90488. However, the technique disclosed in Japanese Unexamined Patent Application, First Publication No. H7-90488 is related to a cold-rolled steel plate having a thickness of 1.8 mm and is premised on a high cooling rate of 70° C./s or greater, so that the technique does not consider weldability.
Hitherto, as a technique for enhancing a delayed fracture resistance of high-strength steel, there has been known a technique of refining grain size. A technique of Japanese Unexamined Patent Application, First Publication No. H11-80903 is an example of this technique. However, in the example, in order to enhance the delayed fracture resistance, the prior austenite grain size needs to be equal to or smaller than 5 μm. However, it is not easy to refine the grain size of a steel plate down to such a size by a normal production process. The technique disclosed in Japanese Unexamined Patent Application, First Publication No. H11-80903 is technique for refining a prior austenite grain size through rapid heating before quenching. However, in order to rapidly heat the steel plate, special heating equipment is needed, so that it is difficult to implement the technique. In addition, due to the grain refining, hardenability is degraded. Therefore, in order to ensure the strength, additional alloy elements are needed. Accordingly, an excessive grain refining is not preferable in terms of weldability and economic efficiency.
For the purpose of wear resistance, a steel member having a high strength corresponding to a yield strength of 1300 MPa-class has been widely used, and there are examples of a steel member taking delayed fracture resistance into consideration. For example, wear-resistant steels having excellent delayed fracture resistance are disclosed in Japanese Unexamined Patent Application, First Publication No. H11-229075 and Japanese Unexamined Patent Application, First Publication No. H1-149921. The tensile strengths of the wear-resistant steels disclosed in Japanese Unexamined Patent Application, First Publication No. H11-229075 and Japanese Unexamined Patent Application, First Publication No. H1-149921 are in the ranges of 1400 to 1500 MPa and 1450 to 1600 MPa, respectively. However, in Japanese Unexamined Patent Application, First Publication No. H11-229075 and Japanese Unexamined Patent Application, First Publication No. H1-149921, there is no mention of yield stress. With regard to wear resistance, hardness is an important factor, so that the tensile strength has an effect on the wear resistance. However, since the yield strength does not have a significant effect on the wear resistance, the wear-resistant steel does not generally take the yield strength into consideration. Therefore, the steels disclosed in Japanese Unexamined Patent Application, First Publication No. H11-229075 and Japanese Unexamined Patent Application, First Publication No. H1-149921 are considered to be unsuitable as a structural member of a construction machine or an industrial machine.
In Japanese Unexamined Patent Application, First Publication No. H9-263876, a high-strength bolt steel member that has a yield strength of 1300 MPa-class is provided with enhanced delayed fracture resistance by elongation of prior austenite grains and rapid-heating tempering. However, the rapid-heating tempering cannot be easily performed in existing plate heat treatment equipment, so that it cannot be easily applied to a steel plate.
In order to enhance the atmospheric corrosion resistance of steel and suppress delayed fracture of bolts, a technique of adding a large amount of Ni is disclosed in Japanese Unexamined Patent Application, First Publication No. 2001-107139. However, since expensive Ni of equal to or greater than 2.3% is added as an indispensable condition, an application to a plate is not practical in view of the cost.
In order to improve delayed fracture resistance by forming protective rust, a technique of adding both Cu and P is disclosed in Japanese Unexamined Patent Application, First Publication No. H8-311601. However, toughness tends to decrease as the amount of P increases. Accordingly, in a high-strength steel plate having a yield strength of 1300 MPa-class, since it is difficult to ensure a balance between strength and toughness, the technique cannot be applied to a steel plate.
As described above, the existing technique is not enough to economically obtain a high-strength steel plate (steel) for a structural member, which has a yield strength of 1300 MPa or greater and a tensile strength of 1400 MPa or greater, and has delayed fracture resistance or usability such as bending workability and weldability.